Pump unit and fluid supplying system

ABSTRACT

A fuel supplying system includes a tank and a pump unit, which transfers fluid from the tank. The pump unit includes a first pump, a second pump, and a drive source. The first pump and the second pump are driven by the common drive source. The first pump, the second pump, and the drive source are structured as one unit. This reduces the size and simplifies the structure of the fuel supplying system.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a pump unit that is used in afuel supplying system of an internal combustion engine that usesdimethyl ether as fuel, and to a fluid supplying system that, which hasthe pump unit.

[0002] A typical pump unit includes a piston pump, which functions as amain source for transferring fluid. To reliably feed dimethyl ether(hereinafter, referred to as DME) from a tank to an internal combustionengine (or fuel injection device) without vaporizing DME, it has beenproposed that a gear pump be provided at upstream of piston pump. Thepiston pump functions as a main source for transferring fluid. The pumpunit reliably feeds dimethyl ether (hereinafter, referred to as DME)from the tank to an internal combustion engine (or fuel injectiondevice) without vaporizing the DME. That is, the DME is compressed inadvance with the gear pump, which has no expansion phase, to prevent thepressure of the DME from decreasing below the saturation pressure by theexpansion (suction) phase of the piston pump.

[0003] However, the conventional fuel supplying system has the twoseparate pumps each having an electric motor as a drive source.Therefore, the size and the cost of the fuel supplying system areincreased.

SUMMARY OF THE INVENTION

[0004] Accordingly, it is an objective of the present invention toprovide a compact and low-cost pump unit and to provide a fluidsupplying system that has the pump unit.

[0005] To achieve the forgoing and other objectives and in accordancewith the purpose of the present invention, the invention includes afirst pump, a second pump, and a single drive source. The first pump hasno expansion phase and draws in and discharges fluid. The second pumphas an expansion phase and draws in and discharges fluid that isdischarged from the first pump. The second pump is connected to thefirst pump. The single drive source drives the first pump and the secondpump.

[0006] The present invention also provides a fluid supplying system. Thesystem includes the above described pump unit and a tank for reservingfluid. The pump unit transfers fluid from the tank.

[0007] The present invention further provides a fluid supplying system.The fluid supplying system includes the above described pump unit, amain tank for reserving fluid, and a sub-tank arranged separately fromthe main tank. The sub-tank receives fluid from the main tank. The pumpunit is attached to the sub-tank to transfer fluid from the sub-tank.

[0008] Other aspects and advantages of the invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention, together with objects and advantages thereof, maybest be understood by reference to the following description of thepresently preferred embodiments together with the accompanying drawingsin which:

[0010]FIG. 1 is a cross-sectional view of a pump unit according to afirst embodiment of the present invention;

[0011]FIG. 2 is a schematic view illustrating a fuel supplying system,which has the pump unit shown in FIG. 1;

[0012]FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 1;

[0013]FIG. 4 is a schematic view illustrating a fuel supplying systemaccording to a second embodiment of the present invention;

[0014]FIG. 5 is a partial cross-sectional view illustrating the pumpunit shown in FIG. 4;

[0015]FIG. 6 is a schematic view illustrating a fuel supplying systemaccording to a third embodiment of the present invention; and

[0016]FIG. 7 is a schematic view illustrating a fuel supplying systemaccording to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A fuel supplying system according to a first embodiment of thepresent invention will now be described with reference to FIGS. 1 to 3.

[0018]FIG. 2 is a schematic view showing a fuel supplying system forsupplying fuel, which is dimethyl ether (hereinafter, referred to asDME) in the first embodiment, to a fuel injection device 101, whichincludes, for example, in-line piston pumps. The fuel injection device101 is located in a drive source of a vehicle, which is a dieselinternal combustion engine (not shown). The fuel supplying systemincludes a tank 11 for reserving DME and a pump unit 12. The pump unit12 is attached to the tank 11 and feeds the DME in the tank 11 to thefuel injection device 101 in a liquid state. The DME is a fluid that isvaporized under the pressure that is less than or equal to thesaturation pressure. In other words, the DME is vaporized at a normaltemperature and under the atmospheric pressure.

[0019] As shown in FIG. 1, the housing of the pump unit 12 includes anupper first center housing 21, a lower second center housing 22, a firstend housing 23, which is secured to the upper end of the first centerhousing 21, and a second end housing 24, which is secured to the lowerend of the second center housing 22. The first end housing 23 of thepump unit 12 is inserted into a hole 11 a, which is formed through thelower part of the tank 11. The upper surface of the first end housing23, or a small part of the pump unit 12, is exposed inside the tank 11.

[0020] The second center housing 22 defines a crank chamber 25.

[0021] A bleed passage 26 extends through the first end housing 23 tothe first center housing 21. The crank chamber 25 is always communicatedwith the tank 11 via the bleed passage 26. The bleed passage 26vertically extends from the tank 11 to the crank chamber 25.

[0022] The second end housing 24 defines a motor chamber 27. A driveshaft 28 is rotatably supported between the first center housing 21 andthe second end housing 24. The drive shaft 28 extends through the crankchamber 25 and the motor chamber 27. A shaft sealing assembly 60 isarranged at the middle portion of the drive shaft 28 and separates thecrank chamber 25 from the motor chamber 27.

[0023] A stator 29 is located inside the motor chamber 27 and is securedto the inner circumferential surface of the second end housing 24. Arotor 30 is located inside the motor chamber 27 and is secured to theouter circumferential surface of the drive shaft 28 facing the stator29. Therefore, the above structure functions as an electric motor, whichis a motor M in the first embodiment. When current is supplied to thestator 29 from the outside, the rotor 30 is rotated, which in turnrotates the drive shaft 28.

[0024] The pump unit 12 includes a gear pump, which is a first pump P1in the first embodiment, and a piston pump, which is a second pump P2 inthe first embodiment. The gear pump has less volume efficiency comparedwith the piston pump and differs from the piston pump in that the gearpump has no expansion (suction) phase. The piston pump has an expansionphase and higher volume efficiency compared with the gear pump.Therefore, the second pump P2 serves as a main pump for feeding the DMEto the fuel injection device 101. The first pump P1 serves as apressurization pump for preventing the DME from vaporizing during theexpansion phase of the second pump P2.

[0025] The first and second pumps P1, P2 shares the motor M as a drivesource. That is, the drive shaft 28 of the first pump P1 and the driveshaft 28 of the second pump P2 are coaxial and uniaxial. The first andsecond pumps P1, P2 and the motor M are surrounded with the housings 21,22, 23, 24 as one unit. The discharge amount of the DME of the firstpump P1 rotation of the drive shaft 28 is set to be equal to or greaterthan that of the second pump P2. That is, the discharge capacity of thefirst pump P1 is equal to or greater than the discharge capacity of thesecond pump P2.

[0026] As shown in FIG. 1, a pump chamber 31 is defined at the jointportion between the first center housing 21 and the first end housing23. The upper end portion of the drive shaft 28 projects inside the pumpchamber 31. A first gear 32 is secured to the projecting portion and isrotated integrally with the drive shaft 28. A second gear 33, whichmeshes with the first gear 32, is arranged inside the pump chamber 31.The second gear 33 is rotated on the same plane as the first gear 32.

[0027] An inlet 34 is formed on the upper surface of the first endhousing 23 above the pump chamber 31. A suction passage 35 verticallyextends through the first end housing 23. The suction passage 35connects the inlet 34 and the low pressure side (left side in FIG. 3) ofthe pump chamber 31. A communication passage 36 extends downward fromthe high pressure side (right side in FIG. 3) of the pump chamber 31through the first center housing 21. The communication passage 36 isconnected to the suction side of the second pump P2.

[0028] When the drive shaft 28 is rotated, the first gear 32 is rotated,which in turn rotates the second gear 33. Therefore, the DME is drawninto the low pressure side of the pump chamber 31 from the tank 11 viathe inlet 34 and the suction passage 35. The DME is then transferred tothe high pressure side of the pump chamber 31 using the space betweenthe teeth grooves of the gears 32, 33 and the inner surface of the pumpchamber 31. The DME that is transferred to the high pressure side of thepump chamber 31 is discharged toward the communication passage 36.

[0029] As shown in FIG. 3, a pressure release passage 37 verticallyextends through the first end housing 23. The pressure release passage37 connects the high pressure side of the pump chamber 31 to the tank11. A relief valve 38, which is formed of a ball valve 38 a and a spring38 b, is arranged in the pressure release passage 37. The ball valve 38a is normally urged by the force of the spring 38 b to close thepressure release passage 37. When the pressure in the high pressure sideof the pump chamber 31, or the pressure in the communication passage 36,is excessive, the ball valve 38 a moves against the force of the spring38 b to open the pressure release passage 37.

[0030] As shown in FIG. 1, the second pump P2 includes a cylinder block39 located inside the crank chamber 25. The cylinder block 39 is fittedto the drive shaft 28 by splines such that the cylinder block 39 isrotated integrally with and relatively moves with respect to the driveshaft 28. Cylinder bores 39 a are formed in the cylinder block 39 aboutthe drive shaft 28. Each cylinder bore 39 a accommodates a piston 40. Acam 41 is secured to the second center housing 22 below the crankchamber 25. An inclined surface 41 a, which is inclined with respect tothe axis of the drive shaft 28, is formed on the upper surface of thecam 41.

[0031] Each piston 40 is coupled to a shoe 43 via a spherical joint 42.

[0032] A valve plate 44 is fixed to the inner end surface of the crankchamber 25 in the first center housing 21. The valve plate 44 includes asuction port 44 a and a discharge port 44 b, each defining an arc aboutthe axis of the drive shaft 28. The cylinder block 39 has a springchamber 39 b formed in the center. The spring chamber 39 b accommodatesa spring 45, which is arranged about the drive shaft 28. The force ofthe spring 45 acts on the cylinder block 39 via a spring seat 46. Theforce of the spring 45 also acts on a shoe retainer via another springseat 47, a pin 48, and a pivot 49. Therefore, the shoes 43 on the shoeretainer 50 are pressed against the inclined surface 41 a of the cam 41and the cylinder block 39 is pressed against the valve plate 44. Theforce of the spring and the force of the cylinder block 39 that isgenerated by the pressure difference between the inside and outside ofthe cylinder bores 39 a and acting toward the valve plate 44 improve thesealing effect between the cylinder block 39 and the valve plate 44.

[0033] The rotation of the cylinder block 39 with the drive shaft 28 isconverted to the reciprocation of the pistons 40. The stroke of eachpiston 40 is determined by the inclination angle of the inclined surface41 a of the cam 41. Each cylinder bore 39 a is alternately communicatedwith the suction port 44 a and the discharge port 44 b of the valveplate 44. Thus, the DME that is pressurized by the first pump P1 isdrawn into each cylinder bore 39 a via the communication passage 36 andthe suction port 44 a. The DME drawn into each cylinder bore 39 a isdischarged from the corresponding discharge port 44 b by a pumpingaction. The DME discharged from the discharge port 44 b is transferredto the fuel injection device 101 via a discharge passage 51, which isformed in the first center housing 21, and an external pipe 13.

[0034] The first embodiment provides the following advantages.

[0035] (1) The first pump P1 and the second pump P2 share the motor M asthe drive source. Therefore, compared to a case where a drive source isprovided for each of the first and second pumps P1 and P2, the size ofthe fuel supplying system is reduced and the structure is simplified,thereby reducing the manufacturing cost. Sharing the drive sourcebetween the first pump P1 and the second pump P2 is advantageous forintegrating the first and second pumps P1, P2 and the motor M as asingle pump unit 12.

[0036] (2) The electric motor M has a simpler structure compared to aninternal combustion engine, or the like. Thus, the first and secondpumps P1, P2 and the housings 21, 22, 23, and 24 can easily beintegrated.

[0037] The electric motor M is also suitable for using as a drive sourceof the pump unit 12 of the fuel supplying system in that the electricmotor M is safe.

[0038] (3) The drive shaft 28 of the first pump P1 and the drive shaft28 of the second pump P2 and the output shaft 28 of the motor M arecoaxial and uniaxial. Therefore, for example, a complex powertransmission mechanism need not be arranged between the motor M and thesecond pump P2, and between the second pump P2 and the first pump P1.Thus, the structure of the pump unit 12 can be simplified, therebyreducing the size of the pump unit 12.

[0039] (4) The relief valve 38 is located in the communication passage36, which connects the discharge side of the first pump P1 and thesuction side of the second pump P2. The relief valve 38 releasesexcessive pressure in the communication passage 36 to the tank 11, orthe upstream of the first pump P1. Therefore, the relief valve 38prevents the pressure from increasing excessively in the communicationpassage 36. This prevents the increase of the power loss of the pumpunit 12 due to the excessive pressure in the second pump P2.

[0040] (5) The crank chamber 25 of the second pump P2 is communicatedwith the tank 11, or the upstream of the first pump P1, via the bleedpassage 26. Therefore, even when the DME that has leaked into the crankchamber 25 is vaporized by the heat generated by sliding parts (such asthe cam 41 and the shoes 43) inside the crank chamber 25, the vaporizedDME returns to the tank 11 through the bleed passage 26. Thus, thevaporized DME inside the crank chamber 25 is prevented from accumulatinginside the cylinder bore 39 a, thereby hindering the suction of the DMEthrough the suction port 44 a. This improves the reliability of the pumpunit 12. The bleed passage 26 also prevents the pressure inside thecrank chamber 25 from increasing excessively.

[0041] The bleed passage 26 extends vertically upward from the crankchamber 25. This structure allows the vaporized DME in the crank chamber25 to reliably float upward to the tank 11.

[0042] (6) The suction passage 35 of the pump unit 12 extends verticallyupward from the pump chamber 31 of the first pump P1. Therefore, evenwhen cavitation occurs are generated inside the suction chamber 35, thevaporized DME floats upward to the tank 11, or the upstream of thesuction passage 35. Therefore, the vaporized DME are prevented frombeing drawn into the first pump P1 or the second pump P2.

[0043] (7) The pump unit 12 is integrated with the tank 11. Therefore,the tank 11 and the pump unit 12 can easily be installed in a vehicle.The tank 11 need not be connected to the pump unit 12 with a pipe.

[0044] (8) The pump unit 12 is almost entirely exposed outside the tank11. This facilitates the maintenance of the pump unit 12.

[0045] A second embodiment will now be described with reference to FIGS.4 and 5. In the second embodiment, the differences from the firstembodiment of FIGS. 1 to 3 will mainly be discussed below with referenceto FIGS. 4 and 5, and like or the same reference numerals are given tothose components that are like or the same as the correspondingcomponents of the first embodiment of FIGS. 1 to 3.

[0046] As shown in FIGS. 4 and 5, the pump unit 12 according to a secondembodiment is accommodated inside the tank 11 and is secured to thebottom of the tank 11. The pump unit 12 of the second embodiment differsfrom the pump unit 12 of the first embodiment in that the pump unit 12is arranged laterally, that is, the drive shaft 28 is arrangedhorizontally. The discharge passage 51 of the pump unit 12 iscommunicated with an outlet 52, which is formed in the bottom of thetank 11. The discharge passage 51 is connected to the external pipe 13via the outlet 52.

[0047] The bleed passage 26 vertically extends through thecircumferential wall of the second center housing 22.

[0048] In the second embodiment, a centrifugal pump is used as the firstpump P1. A bladed wheel 55, which forms the centrifugal pump, is securedto the drive shaft 28 inside the pump chamber 31 and rotates integrallywith the drive shaft 28. Therefore, the bladed wheel 55 is rotated withthe rotation of the drive shaft 28, thereby drawing the DME into the lowpressure side (left side in FIG. 5) of the pump chamber 31 from the tank11 through the inlet 34 and the suction passage 35. The DME that isdrawn into the low pressure side of the pump chamber 31 is thentransferred to the high pressure side (upper side in FIG. 5) of the pumpchamber 31 by the space formed between the adjacent blades of the bladedwheel 55 and the inner surface of the pump chamber 31. The DMEtransferred to the high pressure side of the pump chamber 31 isdischarged toward the communication passage 36 by the centrifugal forceexerted by the rotation of the bladed wheel 55.

[0049] The second embodiment provides the same advantages as (1), (2),(3), (4), (5), and (7) of the first embodiment. The second embodimentfurther provides the following advantages.

[0050] (1) The pump unit 12 is entirely accommodated inside the tank 11.Therefore, the tank 11 has no projections outside. This furtherfacilitates the installation of the tank 11 and the pump unit 12 in avehicle. The bleed passage 26 is shorter than that of the firstembodiment shown in FIG. 1. Therefore, the advantage described in (5) ofthe first embodiment is more efficiently provided. Furthermore, evenwhen the suction passage 35 extends in the horizontal direction, thesame advantage as described in (6) of the first embodiment is provided.This adds to the flexibility of the design of the suction passage 35.

[0051] (2) The centrifugal pump used as the first pump P1 has simplerstructure compared to, for example, the gear pump. Therefore, thestructure of the pump unit 12 can be simplified.

[0052] A third embodiment will now be described with reference to FIG.6. The differences from the first embodiment of FIGS. 1 to 3 will mainlybe discussed below with reference to FIG. 6, and like or the samereference numerals are given to those components that are like or thesame as the corresponding components of the first embodiment.

[0053] In the first and second embodiments, the pump unit 12 is directlyattached to the tank 11. However, in the third embodiment, a sub-tank 61is provided separately from the tank (main tank) 11, which reservesfuel, as shown in FIG. 6. The pump unit 12 is arranged in the sub-tank61.

[0054] The pump unit 12 that is the same as the one that is described inthe first and second embodiments is used in the third embodiment. Thepump unit 12 is secured to the inner bottom surface of the sub-tank 61.More specifically, the first end housing 23 is secured to the innerbottom surface of the sub-tank 61. The first center housing 21, whichincorporates the first pump P1, is secured on top of the first endhousing 23 and the second center housing 22, which incorporates thesecond pump P2, is secured on top of the first center housing 21. Thesecond end housing 24, which incorporates the motor M, is secured on topof the second center housing 22. The bleed passage 26, the suctionpassage 35, and the discharge passage 51 are formed as shown in FIG. 6to be suitable for arranging in the pump unit 12. The discharge passage51 is connected to the fuel injection device 101 by the external pipe13.

[0055] The position of the sub-tank 61 with respect to the main tank 11is determined such that the first pump P1 is arranged lower than theinner bottom surface of the main tank 11. In other words, the inlet 34,which introduces the DME in the sub-tank 61 into the first pump P1, islocated lower than the inner bottom surface of the main tank 11.

[0056] The sub tank 61 is connected to the main tank 11 by a connectingpipe 62. The inlet of the connecting pipe 62 is connected to the bottomwall of the main tank 11 and the outlet of the connecting pipe 62 isconnected to the lower portion of the side wall of the sub-tank 61. TheDME in the main tank 11 is introduced into the sub-tank 61 through theconnecting pipe 62 by its own weight. A return pipe 63 connects theupper wall of the sub-tank 61 (or preferably the uppermost portion ofthe sub-tank 61) to the upper portion of the side wall of the main tank11. Gas is retained in the upper portion of the main tank 11 and theliquid DME does not reach the gaseous space. The return pipe 63 iscommunicated with the gaseous space. The vaporized DME generated in thesub-tank 61 returns to the main tank 11 through the return pipe 63.

[0057] The fuel injection device 101 is connected to the upper portionof the side wall of the main tank 11 by a feedback pipe 64. The feedbackpipe 64 is communicated with the gaseous space in the main tank 11. Theremaining DME that was not injected by the fuel injection device 101returns to the main tank 11 through the feedback pipe 64.

[0058] The third embodiment provides the following advantages.

[0059] (1) When the tank 11 is empty, the tank 11 may be filled with DMEwithout being removed, or the empty tank 11 may be exchanged withanother tank 11 filled with DME. Since the pump unit 12 is directlyattached to the tank 11 in the first and second embodiments, the pumpunit 12 is also replaced when exchanging the tank 11. In this case, thesame number of pump units 12 as the tanks 11 must be provided, whichincreases the cost. In the first and second embodiments, the pump unit12 may be detached from the tank 11 when exchanging the tank 11.However, such process is very troublesome.

[0060] In contrast, the pump unit 12 according to the third embodimentis arranged inside the sub-tank 61 separately from the main tank 11.Therefore, when the main tank 11 is empty, only the main tank 11 iseasily removed from the fuel supplying system and replaced with anothermain tank 11 filled with DME. Thus, the pump unit 12 is not wasted byexchanging the pump unit 12 with the main tank 11 and it is notnecessary to detach the pump unit 12 from the main tank 11.

[0061] (2) The first pump P1 of the pump unit 12 is arranged lower thanthe inner bottom surface of the main tank 11. The DME in the main tank11 flows into the sub-tank 61 through the connecting pipe 62, which isconnected to the bottom wall of the main tank 11, by its own weight. Thevaporized DME retained at the upper portion of the sub-tank 61 returnsto the main tank 11 via the return pipe 63. Therefore, even when thelevel of liquid surface of the DME in the main tank 11 is close to theinner bottom surface of the main tank 11, the DME is reliably sent tothe sub-tank 61 from the main tank 11 and the first pump P1 reliablydraws the DME while being immersed in the DME inside the sub-tank 61.That is, the first pump P1 is able to draw in the DME even when theremaining DME in the main tank 11 is only a small amount. As a result,the DME in the main tank 11 can almost be used up. This is particularlyeffective with the structure that permits the main tank 11 to beexchanged.

[0062] (3) Excessive DME in the fuel injection device 101 returns to themain tank 11 through the feedback pipe 64. The excessive DME is heatedwhile flowing through the fuel injection device 101, but the heat isreleased in the relatively large main tank 11. This suppresses thetemperature increase of the DME and decreases the amount of DME that isvaporized.

[0063] In the fourth embodiment, the differences from the thirdembodiment of FIG. 6 will mainly be discussed below with reference toFIG. 7. In the fourth embodiment, the pump unit 12 is arranged insidethe sub-tank 61, which is separate from the main tank 11, in the samemanner as in the third embodiment.

[0064] As shown in FIG. 7, the pump unit 12 is laterally secured to theinner side surface of the sub-tank 61. That is, the pump unit 12 isarranged such that the drive shaft 28 becomes horizontal. The feedbackpipe 64, which extends from the fuel injection device 101, is connectedto the upper portion of the sub-tank 61 instead of the main tank 11. Thefeedback pipe 64 is communicated with the upper portion, or the gaseousspace, of the sub-tank 61.

[0065] The fourth embodiment provides the following advantages inaddition to the advantages of the third embodiment.

[0066] (1) The feedback pipe 64 is connected to the sub-tank 61 insteadof the main tank 11. Therefore, the number of pipes connected to themain tank 11 is reduced as compared to the third embodiment shown inFIG. 6. This facilitates detaching and connecting procedures whenexchanging the main tank 11.

[0067] It should be apparent to those skilled in the art that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention. Particularly, itshould be understood that the invention may be embodied in the followingforms.

[0068] Instead of DME, cChlorofluorocarbon or propane may be usedinstead of DME asfor fluid that turns into gaseous state under thepressure that is less than or equal to the saturation pressure. That is,the present invention may be embodied in a pump unit that transferschlorofluorocarbon or propane.

[0069] A pump that has no expansion phase includes screw pump and rootspump in addition to the gear pump and the centrifugal pump. That is, thescrew pump or roots pump may be used as the first pump.

[0070] In the above embodiments, the pressure release passage 37 and therelief valve 38 may be omitted. In this case, the discharge amount ofDME of the first and second pumps P1 and P2 per one rotation of thedrive shaft 28 is set to be equal.

[0071] In the above embodiments, the shaft sealing assembly 60 isprovided between the second pump P2 (the crank chamber 25) and the motorM (the motor chamber 27). However, the shaft sealing assembly 60 may beomitted and the motor chamber 27 may be exposed to the DME.

[0072] In the above embodiments, the motor M may be separated from thepump unit 12. In this case, the motor M is connected to and driven bythe drive shaft 28 of the pump unit 12 via the power transmissionmechanism, which includes a belt and a pulley.

[0073] The feedback pipe 64 according to the third embodiment of FIG. 6may be applied to the system according to the first embodiment shown inFIGS. 1 to 3 or the second embodiment shown in FIGS. 4 and 5.

[0074] Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalence of the appended claims.

1. A pump unit comprising: a first pump, which has no expansion phase,wherein the first pump draws in and discharges fluid; a second pump,which has an expansion phase, wherein the second pump is connected tothe first pump for drawing in and discharging fluid that is dischargedfrom the first pump; and a single drive source for driving the firstpump and the second pump.
 2. The pump unit according to claim 1, whereinthe discharge capacity of the first pump is equal to or greater than thedischarge capacity of the second pump.
 3. The pump unit according toclaim 1, wherein the first pump, the second pump, and the drive sourceare coupled with one another to form a single unit.
 4. The pump unitaccording to claim 3, wherein the drive source is an electric motor, andwherein a housing of the electric motor, a housing of the first pump,and a housing of the second pump are coupled to one another.
 5. The pumpunit according to claim 1, wherein the first pump and the second pumpare driven by a common single drive shaft.
 6. The pump unit according toclaim 5, wherein the drive shaft extends to the drive source to servealso as an output shaft of the drive source.
 7. The pump unit accordingto claim 1 further comprising: a communication passage for introducingfluid that is discharged from the first pump into the second pump; and arelief valve for releasing excessive pressure from the communicationpassage.
 8. The pump unit according to claim 7, wherein the relief valvereleases excessive pressure from the communication passage toward asection where the fluid is stored before being introduced into the firstpump.
 9. The pump unit according to claim 1, wherein the second pump isa piston pump, the piston pump comprising: a drive shaft; a piston; ahousing, which defines a crank chamber; a cam arranged in the crankchamber, wherein the cam converts the rotation of the drive shaft intothe reciprocation of the piston; and a bleed passage, which communicatesthe crank chamber with the outside of the housing.
 10. The pump unitaccording to claim 9, wherein the bleed passage communicates the crankchamber with a section where the fluid is stored before being introducedinto the first pump.
 11. The pump unit according to claim 9, wherein thebleed passage extends upward from the crank chamber.
 12. The pump unitaccording to claim 1, wherein the first pump is a gear pump or acentrifugal pump.
 13. The pump unit according to claim 1 furthercomprising a suction passage for introducing fluid into the first pump,wherein the suction passage is structured such that gas that isgenerated in the suction passage can ascend toward the upstream of thesuction passage.
 14. A fluid supplying system comprising: a tank forreserving fluid; and a pump unit for transferring fluid from the tank,wherein the pump unit includes: a first pump, which has no expansionphase, wherein the first pump draws in fluid from the tank anddischarges the fluid; a second pump, which has an expansion phase,wherein the second pump is connected to the first pump for drawing inand discharging fluid that is discharged from the first pump; and asingle drive source for driving the first pump and the second pump. 15.The fluid supplying system according to claim 14, wherein the pump unitis attached to the tank such that substantially almost the entire pumpunit is exposed outside the tank.
 16. The fluid supplying systemaccording to claim 14, wherein the pump unit is accommodated in thetank.
 17. A fluid supplying system comprising: a main tank for reservingfluid; a sub-tank arranged separately from the main tank, wherein thesub-tank receives fluid from the main tank; and a pump unit attached tothe sub-tank, wherein the pump unit transfers fluid from the sub-tank,wherein the pump unit includes: a first pump, which has no expansionphase, wherein the first pump draws in fluid from the sub-tank anddischarges the fluid; a second pump, which has an expansion phase,wherein the second pump is connected to the first pump for drawing inand discharging fluid that is discharged from the first pump; and asingle drive source for driving the first pump and the second pump. 18.The fluid supplying system according to claim 17, wherein the first pumpis located below an inner bottom surface of the main tank.
 19. The fluidsupplying system according to claim 17 further comprising a return pipefor returning vaporized fluid in the sub-tank into the main tank. 20.The fluid supplying system according to claim 17, wherein the fluid isfuel for an internal combustion engine, the system further comprising: afuel injection device for injecting the fuel into the internalcombustion engine; and a feedback pipe, which connects the fuelinjection device to the main tank, wherein the feedback pipe returnsexcessive fuel that is generated in the fuel injection device to themain tank.
 21. The fluid supplying system according to claim 17, whereinthe fluid is fuel for an internal combustion engine, the system furthercomprising: a fuel injection device for injecting fuel into the internalcombustion engine; and a feedback pipe, which connects the fuelinjection device to the sub-tank, wherein the feedback pipe returns theexcessive fuel generated in the fuel injection device to the sub-tank.